Tires are sometimes desired with treads for promoting traction on wet surfaces. Various rubber compositions may be proposed for such tire treads.
For example, tire tread rubber compositions which contain high molecular weight, high or low Tg (glass transition temperature) diene based elastomer(s) might be desired for such purpose particularly for wet traction (traction of tire treads on wet road surfaces). Such tire tread may be desired where its reinforcing filler is primarily precipitated silica which may therefore be considered as being precipitated silica rich.
In one embodiment, the improved predictive wet traction performance for the tread rubber composition is based on a maximization of its tan delta physical property at about 0° C. and a desired low value for rebound at about 0° C.
However, it might also be desired to provide such tread rubber composition containing a high or low Tg styrene/butadiene elastomer (SBR) for wet traction with a lower stiffness at lower temperatures to promote cold weather winter performance, particularly for vehicular snow driving.
In one embodiment, the predictive cold weather performance for the tread rubber composition is based on a minimization of its stiffness physical property at −20° C. (e.g. minimized storage modulus G′).
Therefore, it is desirable to provide such vehicular tire tread with a rubber composition containing high or low Tg SBR elastomers with an optimized (maximized) tan delta property at about 0° C. (for predictive wet traction performance improvement) combined with an optimized (minimized) stiffness property at about −20° C. (for predictive cold weather performance improvement).
It is considered that significant challenges are presented for providing such tire tread rubber compositions that provide a combination of both wet traction and winter performance. To achieve the challenge of providing such balance of tread rubber performances with tread rubber compositions, it is recognized that concessions and adjustments would be expected.
To meet such challenge, it is desired to evaluate rubber compositions:
(A) utilizing a high or low Tg styrene/butadiene rubber,
(B) utilizing a low Tg cis 1,4-polybutadiene rubber,
(C) providing an inclusion of a traction promoting resin in the tread rubber composition to aid in promoting wet traction for the tread,
(D) providing reinforcing filler containing rubber reinforcing carbon black and high content of precipitated silica to also promote wet traction for the tire tread rubber composition, and
(E) providing tris(2-ethyl hexyl) phosphate to promote a lowering of the stiffness of the rubber composition at low temperatures while substantially maintaining a higher stiffness of the rubber composition at the operating temperature range of the tire tread, particularly in a range of about 40 to about 60° C.
Such rubber compositions may contain a petroleum and/or vegetable triglyceride based rubber processing oil to reduce the viscosity of the uncured rubber composition and to thereby promote more desirable processing conditions for the uncured rubber composition. In practice, the high viscosity SBR may be extended with the petroleum based oil or vegetable triglyceride oil in a sense of adding the oil to a low viscosity polymerization cement containing the SBR following polymerization of styrene and 1,3-butadiene monomers to form a composite of oil extended SBR with the petroleum oil or vegetable triglyceride oil before the composite is added to the rubber composition in an internal rubber mixer (e.g. Banbury rubber mixer). Alternately, the petroleum based oil or vegetable triglyceride oil may be added to the rubber composition in an internal rubber mixer to reduce the viscosity of the uncured rubber composition both in the internal rubber mixer and for subsequent rubber processing in a rubber processing apparatus such as, for example, in a rubber extruder.
As indicated, it is considered that significant challenges are presented for providing such tire tread rubber compositions for maintaining a balance of both their wet traction and also low temperature (e.g. winter) performance.
To achieve such balance of tread rubber performances with tread rubber compositions containing the high and/or low Tg diene-based elastomer(s), an application of combinations of petroleum based rubber processing oils and/or vegetable triglyceride type oils, such as for example soybean, sunflower and rapeseed oil, may be used to achieve the desired lower rubber stiffness at low operating temperatures while still promoting a predictive beneficial wet traction based on tan delta measurement at 0° C.
However, while the use of a vegetable triglyceride oil (e.g. soybean oil), has been observed to promote a desirable lower G′ stiffness at about a −20° C. low temperature for the cured rubber composition, it has also been observed to promote a corresponding loss of stiffness of the cured rubber composition at a higher intended operating temperature range (about 40° C. to about 60° C.) of the tire. Such loss of stiffness at such higher operating temperature for a tire tread rubber composition can lead to a reduction of wet and dry handling performance for the tire.
To meet such challenge of providing a silica-rich tread rubber composition containing high and/or low Tg elastomer(s) to promote wet traction combined with promoting a reduction in its stiffness at low temperatures, but maintaining stiffness at higher ambient temperature conditions, it is desired to evaluate the following approach:
(A) replacing a portion of the rubber processing oil (e.g. petroleum based oil and/or vegetable triglyceride oil) with tris(2-ethyl hexyl) phosphate to promote a suitable uncured rubber processing viscosity and to promote a lower cured stiffness of the tread rubber composition at lower temperatures to thereby promote low temperature winter performance for the rubber composition, while maintaining the desired stiffness at higher operating conditions for the rubber composition to thereby promote maintenance of wet and dry handling performance for the tire,
(B) adding a traction promoting resin in the tread rubber composition, particularly at a relatively high resin loading, to promote wet traction of the sulfur cured tread rubber composition,
(C) providing a high content of precipitated silica-rich rubber reinforcing filler to promote wet traction for the cured rubber composition.
In the description of this invention, the terms “compounded” rubber compositions and “compounds” are used to refer to rubber compositions which have been compounded, or blended, with appropriate rubber compounding ingredients. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The amounts of materials are usually expressed in parts of material per 100 parts of rubber by weight (phr).
The glass transition temperature (Tg) of the solid elastomers may be determined by DSC (differential scanning calorimetry) measurements, as would be understood and well known by one having skill in such art. The softening point of a resin may be determined by ASTM E28 which might sometimes be referred to as a ring and ball softening point.